1. Field of the Invention
The present invention relates to a computed tomography (abbreviated hereafter as CT) apparatus for obtaining a tomographic image of an object to be examined by measuring an amount of radiation penetrated through the object to be examined, and in particular, such as CT apparatus adapted to an examination of industrial products.
2. Description of the Background Art
In recent years, various types of a CT apparatus for obtaining a tomographic image of an object to be examined by measuring an amount of radiation penetrated through the object to be examined have been developed for industrial use.
An example of such a conventional industrial CT apparatus is shown in FIG. 1.
This CT apparatus of FIG. 1 is a so called third generation CT apparatus in which the tomographic image is obtained by relative rotational motions of the apparatus and the object to be examined alone, without relative transverse motions.
This CT apparatus comprises a sector scanning unit 101 including an X-ray tube 103 for emitting a fan beam shaped X-ray onto a tire 105 to be examined, and an arm 107 thrusted inside a central bore of the tire 105 which has a plurality of detectors 109 arranged in arc for detecting the X-ray penetrated through one cross section of the tire 105; and a rotation mechanism 111 for holding the tire 105 rotatably around an axis normal to the cross section at a center of the cross section, which also includes a loading mechanism 113 for applying a load pressure on the tire 105 so that the measurement can be performed for the tire 105 in loaded state.
This CT apparatus of FIG. 1 is effective in carrying out the measurement on a ring shaped product, but is also known to have the following problems.
Namely, in a case a high energy X-ray tube is adopted for the X-ray tube 103 in order to increase the energy of the X-ray to be emitted, it is also necessary to use high energy type detectors for the detectors 109.
However, in a third generation CT apparatus, due to the fact that each of the detectors 109 covers a ring shaped data region, i.e., a concentrical area around a scanning center, there is a great possibility of ring shaped artefacts to appear on a displayed image.
Now, in order to prevent the artefacts from appearing, the detectors 109 must satisfy a very stringent requirement regarding a uniformity of their characteristics, and this requires image reconstruction and various corrections such as a sensitivity correction among detection channels (also called an air correction), an off-set correction, and a correction of radiation strength fluctuation (also called a reference correction) to be performed extremely carefully, which in turn makes it very difficult and tedious to obtain a good image quality.
Another example of a conventional industrial CT apparatus is shown in FIG. 2.
This CT apparatus of FIG. 2 is a so called second generation CT apparatus in which the tomographic image is obtained by a combination of relative rotational motions and relative traverse motions of the apparatus and the object to be examined.
Namely, in this apparatus, an X-ray tube 123 for emitting a fan beam shaped X-ray onto a tire 125 to be examined and a detector 129 facing the X-ray tube 123 for detecting the X-ray penetrated through one cross section of the tire 125 are fixedly provided, while the tire 125 is held by a carrier device 131 located between the X-ray tube 123 and the detector 129. The tire 125 is held to be rotatable around an axis normal to the cross section at a center of the cross section and is oscillated back and force in a traverse direction perpendicular to a straight line joining the X-ray tube 123 and the detector 129 while an orientation of the tire 125 is altered for each traverse motion.